This invention relates to loudspeakers, and more particularly to a loudspeaker enclosure and driver system using a transmission-line principle in which waves from back and front sides of a loudspeaker driver are added in phase.
Historically, there have been several ways to mount a loudspeaker driver. One way is to free mount a loudspeaker driver in which both its front and back faces are open and unobstructed. This approach is problematic because of partial cancellation of the front waves by the back waves since they are 180.degree. out of phase.
An alternative to a free-mounted loudspeaker driver is an acoustic suspension loudspeaker in which the driver is mounted on the loudspeaker enclosure's wall facing outward and with its back face emitting into the enclosure. This approach excludes the back waves from coming out of the enclosure and prevents them from interfering with and canceling the front waves, but is inefficient in that it makes no use of the back waves.
To make use of the back waves also, the transmission line concept has been used for many years. An example of such systems is disclosed in U.S. Pat. No. 3,327,808. The back waves inside a loudspeaker enclosure is channeled by a passageway or "transmission line" to bring them out of the enclosure and add to the front waves. The length of the transmission line is tuned such that sound waves traveling through it will undergo a phase shift of 180.degree. so that they will be in phase with the front waves. The back waves can then be advantageously summed to the front waves producing a more efficient loudspeaker.
Prior transmission-line loudspeakers were less than optimum. For example, the back waves emerging out of the transmission line is much attenuated. The large sound pressure differential between the sound pressure levels contributed from the front and back faces of the loudspeaker driver results in less than optimum resultant sound field. The problem is aggravated by the front and back waves being emitting in two different portals resulting in a skewed dipole sound field. Another problem is the turbulence caused by diffractions in the passageway, resulting in variation in path lengths and therefore dispersion of phase shifts and reduced coherence for the back waves.
It is therefore an object of this invention to increase loudspeaker efficiency and sound quality and furthermore to provide loudspeaker systems which do not suffer from said problems.